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freezing behaviour and relative importance of heterogeneous ice nuclei in affecting supercooling of Citrus sinensis fruits were studied. The size of an ice nucleation active (INA) Pseudomonas syringae pool inhabiting fruits was positively correlated with the nucleation temperature (NT) of the plant tissue, with a mean of 3-69 log colony forming units (CFU) fruit"1 at NT~$> —2-5 °C. The INA bacterial pool was responsible for «23% of the nucleation events occurring at this temperature, and «29% were attributed to an additional nucleating source. The latter was sensitive to bacterial ice nucleation inhibitors, it occupied a different microniche from that of P. syringae nuclei, yet was neither a fungus nor any of the bacterial strains ever reported as active. Treatment with an ice nucleation inactive bacterium antagonistic to INA bacteria, a lentil lectin, a protease, and guanidine reduced mean nucleation temperatures (MNT) of fruits to ^ —2-58, —2-66, —4-21, and —4-52 °C, respectively, compared to a MNT^; — 1-67 CC for the controls. Thus, the citrus-associated nucleator apparently contained active proteinaceous components but was void of carbohydrate-like groups reportedly encountered at or near the bacterial ice nucleating site. Despite the different origins of citrus nuclei, bacterial nucleation inhibitors reduced nucleation in the field by 50% at NT^ —2-5 °C, an effect probably exerted through the proteinaceous site possessed by both nuclei.